The hithereto-known electrically conductive substances in which electrons act as charge carriers include, among others, polyacetylene, polypyrrole, polythiophene, polyaniline, polyphenylenevinylene and so on. Among them, polypyrrole provides a comparatively high film strength and has, therefore, been used as the solid electrolyte for solid electrolyte capacitors (Japanese Kokai Patent Publication No. 63-158829). Polyaniline may undergo repeated doping and dedoping electrochemically and has a high degree of dopability as compared with other electrically conductive polymers, so that this compound has been utilized as the positive electrode active material of a secondary cell The 27th Battery Symposium in Japan, 3A05L!.
It is known that these polymers can be produced either by chemical oxidative polymerization using an oxidizing agent (chemical polymerization method) or by electrochemical oxidative polymerization (electrolytic polymerization method).
Chemical polymerization generally lends itself well to mass production. However, the resulting electrically conductive polymer is poor in moldability because it is available as a powder insoluble and/or infusible, and in order that it may be used as an electrically conductive material, the particulate polymer must be dispersed in a coating binder just as it is the case with conductive carbon or metal powders. Thus, there is no merit in the use of such a polymer in lieu of the latter time-honored materials.
On the other hand, electrolytic polymerization gives an electrically conductive polymer in the form of a film. However, while a high film strength can be obtained with certain polymers, typically polypyrrole, sufficient film strength cannot be obtained with others, such as polyaniline and polythiophene. Moreover, as the common drawbacks of the electrically conductive polymers synthesized by the electrolytic polymerization method, the size of the electrically conductive polymer film is limited by the size of the electrode used for electrolysis and, moreover, in order that such a film may be deposited on an insulating support, formation of an electrically conductive precoating layer is essential. Owing to these disadvantages, it has heretofore been difficult to obtain an electrically conductive polymer film or layer having a sufficiently large surface area.
Recently, it has been proposed to overcome these disadvantages by synthesizing a solvent-soluble or thermoplastic electrically conductive polymer by polymerizing a monomer having an alkyl or alkoxy group in the 3-position of a 5-membered heterocyclic nucleus such as pyrrole or thiophene Macromolecules, 20, 212 (1987)!. Among compounds in this category, polythiophene derivatives are very satisfactory in processability but are poor in the stability of electrical conductivity. On the other hand, polypyrrole derivatives are disadvantageous in that the cost of monomers is high and their synthesis involves difficulties in many instances.
More recently, it has been discovered that the polyaniline synthesized by chemical polymerization can be rendered soluble by subjecting it to dedoping Japanese Kokai Patent Publication No. 3-28229!. This polyaniline not only yields a tough film on heat treatment but gives a self-supporting film having a high electrical conductivity on redoping. When these merits are considered in conjunction with the procedural ease of polymerization, this is a very satisfactory polymer. However, when it is coated, for example as an antistatic agent, on a heterogenous substrate, there occurs the problem of poor adhesion or low flexibility and the use of a binder for obviating these difficulties sacrifices the inherent electrical conductivity of polyaniline.
Meanwhile, this soluble polyaniline can be utilized as a positive electrode active substance for secondary batteries. Because this soluble polyaniline can be made available in a variety of forms, electrode design is facilitated compared with the case in which the polyaniline prepared by the conventional chemical or electrolytic polymerization method is used as the positive electrode active substance. However, as the common problem with secondary cells utilizing polyaniline, polythiophene or polypyrrole as the positive electrode active substance, the diffusion of anions associated with charge and discharge is rate-determining and this is a major obstacle to be surmounted for the realization of large capacity secondary batteries. This problem cannot be overcome, either, even when said soluble polyaniline is used as the positive electrode active substance.